1. Field of the Invention
The invention relates to an image processing apparatus and, more particularly, to an image formation apparatus for gradation converting image data including periodicity into data in a print format of printer of an electrophotographic system.
2. Description of the Related Art
In recent years, performance of image apparatuses for inputting, displaying, and outputting an image has remarkably been improved. As typical examples, there are a 12-bits/pixel input scanner, a 10-bits/pixel display, and a 4-bits/pixel output printer.
In such image apparatuses, since image data is handled by a data format adapted to gradation reproducing ability of each apparatus, for example, in the case where the image data inputted by the scanner is printed by the printer, it is necessary to convert the number of gradations of the image inputted by the scanner into the number of gradations which can be reproduced by the printer (hereinafter, such a conversion is referred to as a gradation conversion).
For example, in a printer dependent on an electronic photographic system, a gradation conversion system called a dither method is used. As shown in FIG. 10, according to the dither method, by using a dither matrix in which (N×N) pieces of threshold values are arranged, the threshold values are compared with an image signal, thereby determining ON/OFF for dots.
Since the number of reproducing gradations of the apparatus such as scanner or monitor for inputting the image data to the printer is generally larger than that of the printer, a density of ON/OFF of the dots is locally adjusted by using the dither matrix and the number of gradations of the input image is falsely expressed. Such a dither matrix is classified mainly into a dot concentration type in which the dots concentrate regularly by gradually increasing the threshold value from the center of the matrix and a dot distribution type in which the threshold values are arranged at random.
However, according to the electronic photographic system printer, for example, in the case where the number of output gradations is equal to a binary value (black or white), if the image data including isolated dots such that one pixel is a black pixel and eight peripheral pixels are white pixels is supplied, toner is hard to be stably adhered onto a photosensitive material and a portion of the isolated dot cannot be printed, so that there is a problem such that the gradation of the input data is not reflected to a printed result. Therefore, the dither matrix of the dot concentration type such that the black pixels concentrate on two or four pixels is used.
As an apparatus to which image apparatuses of different gradation reproducing abilities are connected, there is a copying apparatus in which a scanner and an electronic photographic system printer are implemented as shown in FIG. 11. Such a copying apparatus is used for printing data from a personal computer or copying printed matter. Most of the printed matters as targets of the copy have a period of a density change of a light/dense pattern as shown in FIG. 12. It is based on an image format according to printing characteristics or the number of reproducing gradations of the printer and is peculiar to the printer.
In the case where the printed matter having such a period of the light/dense pattern is read by the scanner and gradation converted for use in the electrophotographic system printer, a moire occurs and deterioration in picture quality is caused. The moire is caused by interference between the period of the light/dense pattern included in the printed matter and the period of the light/dense pattern which is formed by the dither matrix of the gradation conversion system.
As one of methods of suppressing such a moire, there is a method disclosed in JP-A-4-104576. According to this method, the periodicity of printed matter is lightened by adding a low pass filter to the front stage of the gradation conversion. However, there are variety of periods as a period included in the printed matter, and in order to obtain a good result for all printed matter, the period included in the printed matter has to be discriminated and coefficients of the low pass filter have to be changed. There is, consequently, a problem such that a scale of circuit which is used for suppressing the moire is enlarged and a load of processes also increases.
As another method of suppressing the moire, as disclosed in JP-A-7-123259, there is a method whereby an error diffusing method is applied to the gradation conversion system. The error diffusing method is a gradation conversion system such that a process of propagating quantization errors caused by quantization based on a threshold value to data which is not quantized is executed on a pixel unit basis, thereby realizing the gradation reproduction of high precision. According to the error diffusing method, however, since isolated dots which cannot be printed by the printer of the electronic photographic system are formed particularly in a low density portion, it is necessary to suppress the isolated dots by combining a template process and other processes. There is also a problem such that in order to prevent causes of deterioration in the picture quality such as a chain-like texture that is peculiar to the error diffusing method, a circuit scale is enlarged and a load of processes also increases.
On the other hand, like “Binarizing method by threshold matrix error diffusing method and memory medium” disclosed in JP-A-10-150565, there is also a system in which by using a blue noise mask as a threshold value in the error diffusing method, various problems of the picture quality which are peculiar to the error diffusion are improved. In a manner similar to the error diffusing method, such a system also has a problem such that isolated dots are formed and the gradation is not reflected to a print result.